1. Field of the Invention
The present invention relates to an image signal processing method and apparatus for processing an input image signal generated by an image signal generating device such as an X-ray CT scanner and MRI and supplying an output image signal to an image forming device such as a laser printer and CRT multi format camera, and more particularly relates to an image signal processing method and apparatus for effecting a gamma correction for the input image signal such that a hard copy having an excellent halftone can be reproduced by the image forming device.
2. Related Art Statement
In order to visualize an image signal generated by an X-ray CT scanner and MRI, there have been practically used a CRT multi format camera and a laser printer for recording a visible image on an image record medium such as photographic film and plain paper. As is well known in the art, a brightness characteristic of a display monitor of the X-ray CT scanner is usually different from a gamma characteristic of an image recording device, so that if the image signal is recorded as it is, a desired halftone could not be obtained on a recorded image. Therefore, there is provided an image signal processing apparatus between the image signal generating device and the image forming device for effecting a gamma correction for the image signal to be recorded. Usually a gamma correction curve of the image forming device such as the multi format camera is set to be substantially identical with the brightness characteristic of the display monitor of the image signal generating device, so that the halftone of the displayed image is faithfully reproduced on the recorded image. However, in a relatively large hospital there are provided a plurality of display monitors of different types and these monitors have different brightness characteristics from each other. Further, even if a plurality of display monitors of the same type are used, the brightness characteristics of these display monitors are usually adjusted in various ways depending upon different requirements of respective doctors in the hospital.
In the above mentioned CRT multi format camera, a gamma characteristic is a combination of a gamma characteristic of a CRT serving as a light source and a gamma characteristic of a film to be used, and this composite gamma characteristic is usually different from that of the display monitor. Moreover, in the known image signal processing apparatus, since the analog image signal is treated, a desired gamma correction curve can be obtained only by a time consuming trial and error method while a plurality of parameters such as contrast, brightness and exposure time are adjusted in various ways. Therefore, it takes a very long time even for an experienced engineer, and sometimes the optimal gamma correction curve could not be attained and the halftone of the image displayed on the X-ray CT display monitor could not be faithfully reproduced on a recorded hard copy.
In the laser printer, the digital image signal is processed and a dynamic range of the output signal is larger than that of the input signal, so that the gamma correction curve can be set rather freely. In the known laser printer, there are provided a plurality of gamma correction curves and an operator selects the most suitable gamma correction curve therefrom. However, in order to select a desired gamma correction curve, it is still necessary to perform the selection in the trial and error method. Further, if the desired gamma correction curve is not prepared, the image signal could not be processed by the optimal gamma correction curve.